In several parts of this specification reference is made to equipment useful for making plastic bags. These products are typically made on equipment which processes a flattened tube of polymeric material, such as polyethylene film, by steps such as sealing, perforating, cutting, etc. With the advent of very thin, high density plastic films and the higher speeds available in modern equipment, it has become important to engineer the components which contact the film to increasingly stringent tolerances.
In other parts of the specification, reference is made to equipment useful for making dropcloths or other coversheet materials. These products are typically made on equipment which processes a continuous lay-flat strip of plastic, again such as polyethylene, by steps such as slitting, folding or gusseting, and winding such material into core or coreless rolls. In the case of thick film which is rapidly wound, it is important to ensure the accuracy of the cut, which tends to bog down as film thicknesses are increased.
For example, stringent tolerances are necessary to maintain the rotary fly-knife in a straight condition as it contacts the bed knife in machines which employ such techniques for cutting and/or perforating bags or coversheet materials. No matter how carefully the knife blades, their support beams and bearing and drive components are manufactured, however, bounce and deflection can and do take place at the high rotation speeds encountered (e.g., 300 and higher feet per minute, circumferential speed). With film thickness of 1 mil or lower, and with even the most careful attention to manufacturing detail, problems are regularly encountered which prevent getting a good cut against the bed knife. Obviously, if for any reason the knife is not precisely set at various locations along its length, either the knife will be damaged by considerable striking against the bed knife or will fail to cut entirely through the film being processed. Moreover, the condition of the knife may change with speed, temperature and other factors. When the knife is not properly set, repair, adjustment or replacement is necessary, leading to the costs associated therewith, as well as machine and personnel downtime.
The types of problems just mentioned also occur, with varying degrees, in other types of machinery which involve the use of elongate rolls. Such applications include, but are not at all limited to, paper manufacturing, paper converting (e.g., die cutting for folding cartons or corrugated containers), printing, other applications including film handling (foil, paper, plastic and the like), to name a few. Accordingly, throughout this specification, wherever references are made to the rotating fly-knife in a rotary bag making machine, it should be understood that each of the other types of devices referred to above and others could also benefit from the teachings hereof.
The assignee of the present invention has been instrumental in the development of high speed rotary bag making and winder equipment. A machine which will benefit from the present invention is illustrated in Gietman, U.S. Pat. No. 4,642,084, issued Feb. 10, 1987 for "Plastic Bag Making Machine". The particular machine shown therein will be helpful for describing one environment in which the present invention has great applicability. The bag machine includes a rotary sealing drum having one or more sealing bars located around the drum periphery. A blanket is mounted on a plurality of rollers located about the drum to provide circumferential contact between the drum and blanket. Drum motor means are coupled to the blanket rollers to drive the drum through the frictional contact between drum and blanket.
An incoming, flattened tube of plastic film, e.g., polyethylene, is fed into the drum-blanket nip and through the circumferential area of contact where it is contacted by a sealing bar to create a transverse seal across the film. In the device illustrated in the patent, the sealed film stock exits the drum area and is folded over a plurality of folding boards to reduce the width of the film to approximately 1/4 its original width (greater or lesser folding, of course, can be accomplished at this stage). Following folding, the film stock is fed by a plurality of feed rollers into the nip of a rotating fly-knife and a stationary bed knife to perform a cutting and/or perforating operation, such that the film stock at this point includes a continuous line of connected bags which can be separated one from another by tearing across the perforation line. Downstream of the knife system the bags can be separated and folded, or they can be wound into core or coreless rolls, procedures which in and of themselves do not pertain to the present invention.
One aspect of the earlier Gietman invention which is of note here is the driving system which results from the original motor-blanket roller pulling drive. Since all blanket rollers would be driven by the blanket at the speed determined by the motor, thus establishing the drum speed, and thus defining the line speed for the film being procured, it is shown that a belt and pulley system coupled to a pulley in the motor is coupled to a gear box, in turn coupled by another belt and pulley system to a variator device. The gear box permits downstream operations to be varied in integral adjustments, depending on the number of sealing bars activated on the drum. The variator permits fine tuning to ensure that the cut/slit operation and sealing operations are occurring at the proper locations. It should also be mentioned that the drum diameter in the aforementioned device can be varied to form a widely differing variety of bag lengths.
Coupled between the variator and the fly-knife of the earlier system is yet another belt and pulley drive system which rotates the fly-knife at the desired speed. With the difficulties in maintaining straightness, and the deflection problems which are being experienced with wider machines and with increasing speed (noted above), improvements are needed in this area of the machine. Widths of up to 70 inches are now encountered in this type of equipment and films as thin as 0.003" are now being used. As speeds vary, the deflection problems vary, thus resulting in adjustment problems, no matter how carefully the engineering is performed.
The assignee of the present invention has also been instrumental in the development of coreless winders for strips of pliable materials. Another machine which benefits from the present invention is illustrated in Gietman Jr., U.S. Pat. No. 4,695,005, issued Sep. 22, 1987 for "Coreless Winder for Strips of Pliable Material." The machine disclosed therein will be helpful for describing another environment in which the present invention has applicability.
The winding apparatus disclosed therein includes a dancer system for "slaving" the winder to the sheet material feed device (e.g., an unwind station or a film extruder). The sheet material enters a pull roll section, having a specified roll diameter, which cooperates with a stationary bed knife and a rotating fly knife to permit accurate separation of the feed material into strips having the desired length. A first conveyor section follows the cut-off section and leads to a pivoting conveyor section, the latter being adapted to selectively direct the sheet material to one of a plurality of winding assemblies.
Each winding assembly preferably features dual rods, the rods being spaced apart to receive the leading edge of the sheet material to be wound. The winding assemblies also include systems for causing rotation of the rods to effect the winding of the sheet material and a mechanism for collapsing one of the two rods toward the other, whereby the inner wraps of the sheet material are loosened to permit easy removal of the roll. A push-off palm may be employed to assist in roll removal.
Widths of up to 12-15 feet are now encountered with this type of equipment and films as thick as about 4 mils are used. As speeds increase, maintaining the accuracy of the cut becomes increasingly difficult.
Problems similar to these described above have been noted in several other devices for processing strips of pliable material. See, for example, Belongia, U.S. Pat, No. 4,553,461, issued Nov. 19, 1985 for "Rotary Web Processing Apparatus". In that device, preloaded rollers tend to control roller bounce and maintain constant the gap between rollers. In the disclosed embodiment, such pre-load rollers are located at either end of a roller.
Hirsch, U.S. Pat. No. 4,364,293, issued Dec. 21, 1982 for "Rotary Knife Holder With Means For Dampening Its Natural Frequency Oscillations", employs a cylindrical mass whose ends are surrounded by elastic annuli installed in a sleeve fixedly mounted in the axial bore of the carrier. The frequency of the system is attuned to the natural frequency of the carrier and is installed midway between the ends of the carrier.
Tornberg, et al., U.S. Pat. No. 3,222,966, issued Dec. 14, 1965 for "High Speed Web Punching Device", use a drag roller urged into position by other rollers bearing against it to force the drag roller into position. The tensioning wheels used therein are in constant contact with the roller and are spaced along its length.
Teplitz, in U.S. Pat. No. 2,850,092, issued Sep. 2, 1958 for "Flying Shear", uses counter weights built into knife-carrying rolls to help maintain balance.
None of the aforementioned patents describe or teach a system for maintaining precise alignment along the length of a rotary member, such as a rotating fly-knife of the type described in the beginning portion of this section. A system which would do so would represent a significant advance in the art and address a need long felt by those skilled in the art.